1. Field of the Invention
The present invention relates to a woven fabric for industrial materials which is prepared from a thermoplastic polymer composition primarily comprising polylactic acid or a copolymer of lactic acids and the other hydroxycarboxylic acids than lactic acids (generic name of the polylactic acid and the copolymer will hereinafter be referred to simply as a lactic acid base polymer) and can be degraded under natural environment, and a composite sheet fabricated from said woven fabric. Further, the invention relates to flexible containers, water shielding sheets and resin coated-cloth tubes for ventilation (or resin coated-cloth ventilation tubes) which can be obtained using the composite sheet.
2. Related Art of the Invention
Woven fabrics of synthetic resin filaments which have been developed for industrial uses are plain weave fabrics, twill weave fabrics and satin weave fabrics of nylon, polyester, polypropylene and polyethylene. These synthetic resin fabrics are excellent in mechanical strengths and thus widely used for mesh sheets in a site of building and construction. Further, composite sheets prepared from these fabrics are used for flexible containers, water shielding sheets and resin coated-cloth ventilation tubes in a construction site. These industrial materials are excellent in folding ability, air leakage resistance, waterproofness, welding ability, weatherability and flexibility, and are hence useful for a site of building, construction and mining.
Although woven fabrics made of the above synthetic resins and a composite sheet fabricated from the woven fabric have some excellent properties, refuse disposal of them causes environmental problems. Because these materials are made of synthetic resins which are not degraded almost under the natural environment, they have a very slow rate of degradation and thus remain semipermanently when discarded as refuse and buried under the ground. Disposal of these plastics in the ocean causes damage of a view or destruction of the living environment of marine organisms.
On the other hand, polylactic acids and a copolymer of lactic acid and the other hydroxycarboxylic acids than lactic acids have been developed as thermoplastic polymers having degradability. These polymers can be completely biodegraded within a few months to an year in an animal body. When the polymers are placed in soil or sea water, they start to decompose within a few weeks in a moist environment and disappear within several years. Further, final degradation products of these polymers are lactic acid, carbon dioxide and water which are nontoxic to human body.
Polymers of lactic acid are usually prepared from a cyclic dimer of lactic acid which is called lactide, and U.S. Pat. Nos. 1,995,970, 2,362,511, and 2,683,136 have disclosed a polymerization process of lactide. U.S. Pat. Nos. 3,636,956 and 3,797,499 have disclosed a process for copolymerizing lactic acid and glycolic acid. In the copolymerization of lactic acid and other hydroxy carboxylic acid copolymer, lactide and a cyclic ester intermediate, for example glycolide, i.e., a dimer of glycolic acid, are used for polymerization.
Although the above degradable polymers are widely used for medical materials, for example, suture, absorbent cotton and so on, woven fabric of the above degradable resins and a composite sheet fabricated from the woven fabric have been unknown for use of many industrial materials.